Do I have it correct that I should be using 10/2 w/ground on a 30A circuit?

I'm going to run new cable in conduit out to my shop, from my house. The current circuit is a 20A single pole with 12/2 UF running approx 40 feet to the shop. I notice when I have the lights on, and start the table saw or air compressor (not commercial grade), the lights dim pretty badly. I'd like to alleviate this by bumping up the service out there. I won't be running anything more than 120V, but I'd like to be able to have some overhead for when I'm running multiple machines.

Does this sound reasonable?

My plan is to put a 30A breaker where the 20A is now, and run 10/2 w/ground UF cable to the exterior wall, go through the sill to a weatherproof box, then down through buried conduit all the way to the shop, coming up the outside wall and through the wall of the shop...all in plastic conduit. From there, I would place a junction box. The 10/2 would be split at that point...one branch for lighting, the other for outlets.

Any thoughts on this? I'd run a feeder out there, but that would add significant cost, and my main panel is already full...no room for another double-pole breaker.

Generally speaking NO. You CANNOT have 120v general use receptacles on a 30A breaker.

Either run a feeder to a small sub-panel, or run the 10/2 as you suggest but LEAVE the 20A breaker.

I'd go the feeder route. I would not waste my time and $$ on the #10.

+1. As long as you are digging for the conduit, make it a bit bigger. Run some #6 feeders out there and put in a small 60 amp panel. Then you could have more than one circuit feeding all the toys in your shop. And as long a you have the trench open, you might want to consider a second conduit for coaxial cable and telephone lines. You may not plan on having that, but if you change your mind in the future, then you won't have to dig again.

Good advice, thank you. The feeder is the optimal choice, but as mentioned, with my 100A service, my main panel is already full (save for that one spot), and already has one sub-panel next to it. I suppose I could make an opening if I had to. A lot of this would be solved with 200A service! Might have to bite the bullet next year.

You are not suffering a voltage drop at 45 feet. You just need an extra circuit for your lights and that beer fridge. I think a 10-3 w/grnd would suit you fine. You could have a small sub panel and have a 20amp for saw and a few other things too. A lot of people are thrown off by the amount of breakers in a panel when trying to determine their loads. If you could tell us what's in your house for breakers and what they do, like an oven, dryer, hot water, heaters, well pumps, ect. ect. and how big your house is and what size is sub panel and what's in it? If we knew these things we could probably help you in the right direction. Good luck....

No, not voltage drop, just too many things on one circuit. Actually, I've never had it trip the breaker, but I would just feel more comfortable with some overhead. Plus, I may be running a small heater out there next winter...nothing major (still 120v), and it would be nice to have juice for that. I'll see what I can come up with for an overview of my current setup with some photos.

Plus, I may be running a small heater out there next winter...nothing major (still 120v),

Why is it folks have this mindset that 120v loads are nothing, but "220" means BIG stuff???

A 120v heater IS big stuff. About as big as you can put on a typical 120v circuit. They can be the source of many issues such as overloading existing circuits in old homes.
Even your 120v saw is a BIG draw. Why do you think you are getting these dimming issues.

A small 240v load is easier on wiring than a moderate to large 120v load.

How about running a 20A multi-wire circuit out there and converting the saw over to 240v? Is your saw capable of this?

Why is it folks have this mindset that 120v loads are nothing, but "220" means BIG stuff???

A 120v heater IS big stuff. About as big as you can put on a typical 120v circuit. They can be the source of many issues such as overloading existing circuits in old homes.
Even your 120v saw is a BIG draw. Why do you think you are getting these dimming issues.

A small 240v load is easier on wiring than a moderate to large 120v load.

How about running a 20A multi-wire circuit out there and converting the saw over to 240v? Is your saw capable of this?

Exactly. You're much better off getting a 240v heater. There's much less load on the conductors.

The fact that you already have a subpanel shouldn't restrict you from putting a small panel in your shop. There's no reason you couldn't add another one. I still think that's your best bet.